The invention relates to a device for supplying fuel at a regulated flow rate to an engine, in particular an airplane gas turbine engine.
A device for supplying fuel to a gas turbine engine usually comprises a volumetric pump driven by the engine via an accessory gear box coupled to a shaft of the engine. The volumetric pump receives fuel coming from a fuel circuit of the airplane. An electro-hydraulic controlled metering valve is mounted in a supply pipe connecting the outlet from the volumetric pump to a combustion chamber of the engine. A fuel return circuit with a controlled variable-restriction bypass valve is connected between the outlet and the inlet of the volumetric pump. The bypass valve is controlled hydraulically to maintain the head loss through the metering valve at a value that is constant or almost-constant, enabling fuel to be delivered at a desired rate corresponding to the position of the metering valve. An engine over-speed or over-thrust valve can be mounted in the supply pipe in series or in parallel with the metering valve to cause the fuel flow rate to be decreased in response to detecting an excessive speed or thrust that might arise due to a failure of the metering valve or of its control. A cutoff valve is generally provided in series with the metering valve and the over-speed valve to turn off the engine by interrupting its fuel supply on direct command from the cockpit. Reference can be made in particular to documents EP 1355054 and US 2004/0117102.
In the embodiment of document EP 1355054, the over-speed or over-thrust valve is a valve having two operating states: a fully-open state under normal conditions, and a state of predetermined greater restriction in response to over-speed or over-thrust being detected. Such a disposition presents the drawback, after over-speed or over-thrust has been detected, of losing thrust from the engine or of imposing a reduced volume flow rate of fuel that might not be acceptable for the engine under certain operating conditions.
Document EP 0377292 describes a device for supplying fuel to a gas turbine. The fuel (a compressible gas) is delivered by a compressor that regulates its flow rate under normal conditions. Two valves are mounted in series in a supply pipe between the gas compressor and the injectors of the turbine. Those valves are in the fully-open position under normal conditions to avoid head losses and they are controlled under transient conditions (when starting) or in the event of sudden off-loading to regulate respectively the pressure and a reduced flow rate.
Document U.S. Pat. No. 4,280,323 describes a device for supplying a gas turbine with fuel that includes an over-speed valve and a regulator valve connected in series. The pressure of the fuel delivered by the pump is increased so as to conserve a desired pressure at the injectors of the turbine in spite of the fuel having greater viscosity when cold (on starting). That action is simply of the type that is programmed to match a correlation relationship, and has no regulation loop.